Your poor little boy. I am so sorry to hear that this burden is on his mind. It must have been very hard for him to tell you. I have a 5 year old as well and I know she would be having similar upset. I will be the last one to get on your case about trusting your 5 year old, not my place. Our flock belongs to my 5 year old, so she is trusted with all of it and they make mistakes like everyone does.
Sorry, I took so long to reply, I was out at work... I am not sure what action you have taken to this point. Certainly you can consider culling if it seems the chick is in pain or is deteriorating or you think will be too disabled to continue, but I usually operate on the "if they seem ok I am willing to give them a chance" model. You will have to balance that with how much time and energy you are able to put into this chick.
Regarding the splint, I think you would have to splint the entire leg or nearly so. The vet wrap was an inspired idea. Perhaps you could splint the middle and use the vet wrap to hold it in position? If the leg is that twisted it is possible that it is both broken and dislocated. I am not sure how else you can stabilize the brace. If the tube is the full length of the leg it will certainly make it challenging for it to move around, but it might be the only option to fully stabilize the joint. If it is in the hock (is that the right chicken anatomy term?), you will need to support both sides to keep it from twisting and you will need to have it about 1/2 way up the thigh and 1/2 way down the calf for it to be effective. If the twisting is coming from the hip, that is a whole other thing and I don't have a human to chicken brace that comes to mind. I think you can safely play around with some tube lengths until you find the right one. Bone remodeling, even in a growing chick, would take more than a few days to complete, so you still may be within the window of repair. The leg will never be what it was, but again, I have seen all kinds of creatures get along just fine with less.